Piezoelectric micromachined ultrasound transducers (PMUTs) are making rapid strides in terms of application development in varied areas such as proximity sensing, gesture recognition, medical imaging, photoacoustics, etc. Our ability to fabricate these devices more reliably and integrate them in suitable form factors is paving the way for new and promising applications. PMUTs are now expanding in terms of their fabrication technology as well, with the introduction of new active materials such as AlN and ZnO. In particular, AlN based PMUTs hold great promise because of their CMOS compatibility. We have been exploring PMUT fabrication using three different materials — PZT, AlN and ZnO — and integrating them in fluidic environments for developing fluid spectroscopy.

Probing fluids in very small volumes for detecting changes in their properties such as density, viscosity, and homogeneity can pave the way for online and in-situ monitoring of various fluids in applications as wide ranging as petrochemical industries to medical emergency care. In most health monitoring applications, whether for machines or humans, the emphasis is on early detection of onset of undesirable conditions. For fluids, minute but persistent changes in their density and viscosity can be used as signatures for such onsets. PMUTs hold promise for such detection because of the ease with which they can scan for these changes. We have carried out experiments with PMUTs in different fluids to find spectral shifts with change in their density and viscosity. In this paper, we show how these shifts are quantitatively related to density variation. We report a density sensitivity of 145 Hz/(kg/m³) with a 500 kHz PMUT. We also show some initial results with an array of PMUTs of different frequencies — a new technique we are developing for fluid spectroscopy. PMUTs spanning a range of frequencies and embedded in microfluidic channels are intended for probing fluid inhomogeneity due to particulate suspensions or variations in discrete constituents such as cells in biofluids. Our PMUTs in these studies employ the same material stack for the individual PMUT-cell structures with thin film PZT as the active material, and the frequency variation is achieved simply from geometric scaling. This approach provides great flexibility in terms of building frequency-pixelation of desired resolution. The study of the relationship between frequency-pixelation and the resolution of inhomogeneity in fluids does not exist and is currently underway in our research group.